1. Field of the Invention
The present invention pertains to bow correction techniques within elements in linear arrays and, more particularly, to more efficient designs that provide increased bow correction while requiring less circuit interconnects than previous designs.
2. Description Relative to the Prior Art
The use of mechanical placement devices to arrange elements in linear arrays results in inherent deviations from true linear placement. The types of elements that are typically placed into linear arrangements can be those that receive data, as well as those that transmit data. Among these are full width scanner, full width ink jet printers, as well as electrostatic printers. Ink jet and electrostatic printers need to place LEDs and their associated drivers in linear arrays. Full width scanners need to place sensors into linear arrays. In each of the aforementioned cases, there are prior art references detailing methods and apparatuses for increasing the linearity of elements used within these devices. The description that follows deals specifically with electrophotographic arts. However, it should be understood that similar or identical issues are relevant to the additional types of receiving and transmitting elements requiring linear placement.
In electrophotographic arts, there is inherent process variability in printhead manufacturing and lens assembly, resulting in individual LED exposure paths that are not directly in line with each other in what is commonly termed the in-track direction (IT). Several causes are: variability of LED placement on the chip; skewed LED chip placement on the substrate; non-linearity of lenses; variability of lenses; inherent bow in lenses; mechanical mounting processes, as well as other causes. The term xe2x80x9cbow,xe2x80x9d as used within this document, references a curved line of LEDs. Ideally, of course, it is desired to have no xe2x80x9cbowxe2x80x9d effect on an LED exposure plane, as shown below in FIG. 2a; that is the exposure plane is perfectly straight. This is particularly crucial for a multi-station printing device, or multiple color tandem machine, since printheads with similar direction and magnitude xe2x80x9cbowxe2x80x9d need to be used within the same machine in order to provide acceptable color-to-color registration.
After the electrophotographic writers have been assembled with the optics, the pixel deviation of the writer (from a straight line) can be measured on the image plane. In many cases, the pixel position deviation from the in-track direction (bow) can be more than the pitch of the pixel (for example for a 600 dpi system, the bow can be larger than {fraction (1/600)} inch). Prior art teachings have shown that this type of pixel position deviation can be corrected electronically using digital circuitry on the printhead writer. One such disclosure is U.S. Pat. No. 5,585,836, issued to Pham et al. The teachings of U.S. Pat. No. 5,585,836 are useful in reducing color-to-color misregistration that occurs within a tandem printing machine and also to correct misalignment in printhead systems having a pixel deviation within elements of the printhead writer. This prior art document illustrates an electronic adjustment to rearrange the electronic printing data that is misaligned into the proper pixel line. However, the circuit provided to correct the misalignment of pixels results in numerous circuit interconnects which must be carried through the printhead board to the LED drivers. Also, the disclosure of U.S. Pat. No. 5,585,836 provides for a pixel alignment correction circuit design that employs numerous semiconductor devices that, in total, use a large amount of space.
FIG. 2b is an example of LED misalignment in a printhead due to positional variations across the length of the printhead. The circles represent the ends of the LED chip arrays. Note that this particular printhead contains two separate bow effect areas. There is a negative bow curve in the first half (left side of graph) of the writer LED placement measurements, then the center comes back close to desired, then another negative bow curve is measured in the second half of the writer. This is just one writer examplexe2x80x94any shape and combination of shapes is possible from one device to the next. Current alignment methods specify very tight tolerances on LED locations by: measuring incoming LED placement locations, sorting the printheads in accordance with the resulting bow, and then selecting printheads having similar bow characteristic to be used in the same machine. Referring, again, to FIG. 2b, which shows a rather unique dual bow shape, the possibilities of matching unique characteristic shapes with multiple other printheads becomes increasingly difficult. The cost of sorting printheads for manufacturing, inventory and service is extremely high and the logistics are very difficult. There remains, therefore, a need that will correct mechanical placement errors that are within a single pixel pitch. There further remains a need for a circuit that can provide electronic delay circuitry that can provide pixel pitch correction within a single pixel pitch, or sub-pixel pitch, that employs fewer circuitry elements.
The present invention addresses the aforementioned problems within the prior art by increasing the linearity at which the LED elements to a printhead expose a receiver. Assembled writers have pixel alignment that deviates from a straight line that can be measured on an image plane. In many cases, the pixel position deviation in the in-track direction (bow) can be more than the pitch of the pixel. The present invention addresses this type of pixel position deviation electronically to reduce color-to-color misregistration in tandem printing machines. The present invention applies a coarse electronic adjustment to rearrange the electronic printing of data into the proper pixel line and then a fine electronic adjustment is made to pixels to get the bow error to further reduce to a fraction of a pixel line. Delays of exposure control signals are used by the fine electronic adjustment to correct linearity by a fraction of a pixel line. The delays can be repeated to multiply the number of delays available and increase the linearity resolution. The delays can also be averaged between odd and even rows of elements to increase apparent resolution. The first embodiment employs digital circuitry to provide the fine adjustment of pixel data that is partially located on the interface board and partially located on the printhead substrate. This reduces the amount of circuitry placed within the ASICs on the printhead substrate. The fine adjustment circuitry of first embodiment provides multiple signals on a single circuit trace between the interface board and the printhead substrate, where each of the multiple signals are active at different times. The second embodiment places all the fine electronic adjustment onto the ASICs within the printhead substrate, resulting in fewer interconnects between the interface board and the printhead substrate than the first embodiment.
The first embodiment of the present invention employs digital circuitry to provide the fine adjustment of pixel data that is partially located on the interface board and partially located on the printhead substrate. This reduces the amount of circuitry placed within the ASICs on the printhead substrate. In the art of electrophotographic printing, a printhead can have thousands of LEDs mounted in a small space. These LEDs are connected to driver circuitry, which is, in turn, connected to additional electronics. The connections on the printhead writer are made through numerous wires that are finer than a human hair. The complexity of a modern printhead dictates a large number of connections. These signals must then be carried throughout the printhead itself, resulting in additional signal traces and associated interconnects. The first embodiment of the present invention alleviates this problem by providing a printhead apparatus that creates the necessary signals on the interface board and within ASICs on the LED substrate.
The second embodiment of the present invention uses fewer interconnects between the interface board and the printhead substrate than the first embodiment. This result of fewer interconnects comes at the expense of an increase in the amount of circuitry placed within the ASICs on the printhead substrate. This results in fewer signals that must then be carried throughout the printhead itself. However, the result is that additional circuitry must be placed in the ASICs on the printhead substrate itself. The second embodiment of the present invention provides a printhead apparatus that creates the necessary signals on the printhead substrate within ASICs that provide the LED drivers themselves, thereby eliminating the need to import these signals from the electronic circuit board that interfaces with the printhead.
These and other objects are provided by the present invention by a bow correction circuit for the linear arrangement of elements comprising: a substrate assembly having a plurality of elements, each having associated driver subassemblies, each of the elements representing a pixel within a line; an interface board coupled to the substrate assembly, the interface board having circuitry that processes image data for the elements; a course bow correction circuit on the interface board that electronically arranges the pixels to improve linearity by integral numbers of pixel pitch; and a fine bow correction circuit located at least partially on the substrate, the fine bow correction circuit providing a first circuit common to a plurality of the elements and a second circuit dedicated to a specific element, the second circuit selecting one of a set of delays that improves linearity of the pixels within the line by a fraction of a pixel pitch.